This invention relates to a method and means for preventing or minimizing the deposition of scale from hard water on water wet surfaces in evaporative cooling systems.
In the operation of an evaporative cooling system, water is lost from the system by evaporation which helps to cool the remaining water. As water is lost by evaporation, make-up water is added to maintain the required water inventory in the system. At the same time, however, dissolved minerals are left behind by the evaporating water so that as more and more water is lost by evaporation, the concentration of dissolved solids in the remaining water grows greater and greater. Eventually, the concentration of dissolved solids reaches and exceeds the saturation level with the result that solids begin to precipitate out of the water solution, and deposit as scale on the heat exchange surfaces associated with the evaporative cooling system. Scaling on heat transfer surfaces decreases the rate of heat transfer. The loss of heat transfer efficiency leads not only to increased operating costs but also to deterioration of equipment.
A common method of preventing the build-up of dissolved minerals in the water of an evaporative cooling system involves the addition of chemicals, e.g. ethylenediaminetetraacetic acid, to maintain the scale forming solids in solution, periodically draining a portion of the water containing dissolved scale and adding fresh make-up water. Many proposals for preventing scale deposition have been made. A leading method involves chemical treatment of the water, measuring the conductivity of the water in the system, draining water containing dissolved solids from the system in response to the conductivity measurement when the preset limit on dissolved solids is reached and supplying fresh water and chemicals to the system. Even will such a method, there is a need for one which does not require as much drainage of water from the system and therefore lower chemicals consumption. As described in greater detail hereafter, this invention provides a reliable method of limiting the build-up of dissolved minerals in the water of an evaporative cooling system to a predetermined level and at the same time restricting the drainage of water. The method is automated by a reliable but comparatively inexpensive means for carrying out the control method.